Positron emission tomography (PET) is an imaging method which is mainly used in medical imaging. In the process images of living organisms are produced in which the distribution of a previously administered weakly radioactively marked substance is made visible in the organism, the substance having been enriched in the organism such that biochemical and physiological processes can be imaged. The substances are called radiopharmaceutical agents, with radionuclides, which emit positrons on decomposition, being suitable in particular. After a short distance, by way of example 2-3 mm, the positrons interact with an electron and what is referred to as annihilation occurs. In the process both particles, positron and electron, are destroyed and two high-energy photons are produced with energy of 511 keV each. These photons distance themselves from each other at an angle of about 180° and can be measured, by way of example, on a detector ring where they simultaneously meet at two points. Evidence of positron emission and an estimation of the location of annihilation is possible due to the coincidence of the two measuring results.
However, it should be noted in this connection that the photons emitted by the radiopharmaceutical agent are partially absorbed by the tissue surrounding them or other substances in the imaging region. This attenuation must be taken into consideration in the image reconstruction since image artifacts that are disruptive in other respects are produced. It is known in this regard to determine a data record of attenuation values in the target area, frequently called an attenuation map or p-map. The attenuation map contains the specific absorption for each image voxel and is used for correction.
PET is often used together with other imaging modalities which may then also be used to determine the attenuation map. Thus, by way of example in the case of combined PET computerized tomography (CT) systems this data record is produced from a CT data record which is measured before the PET examination. The attenuation in the case of computerized tomography (about 120-140 kV tube voltage) is converted into the PET attenuation (511 keV), it being possible to optionally employ additional transformation steps, by way of example smoothing filters, segmenting and the like, as is known from the prior art.
Recently, however, PET has been combined with magnetic resonance (MR) in which the examined object is positioned in an MRI device in a comparatively static homogeneous basic magnetic field, so the nuclear spin thereof. is oriented along the basic magnetic field. High-frequency excitation pulses are beamed into the examined object to activate nuclear spin resonances, the activated nuclear spin resonances are measured and MR images reconstructed on the basis thereof. Fast-switched magnetic gradient fields are superimposed on the basic magnetic field for the purpose of site coding. Magnetic resonance allows excellent depiction in particular of soft tissues with selectable contrasts.
To be able to use the advantages of PET and magnetic resonance in combination, combined PET-MR devices have been proposed, wherein, however, there is the problem of determination of the attenuation map being more difficult in this case since there is initially no correlation between the MR signal strength and PET attenuation. Various methods are known, however, in order to still be able to derive an attenuation map from a magnetic resonance data record, the magnetic resonance data record being registered, by way of example, on an atlas describing the target area and in which corresponding attenuation values are allocated to various anatomical structures to thus obtain an attenuation map with which the PET image data records may subsequently be corrected. A procedure of this kind is known, by way of example, from DE 10 2004 043 889 A1. The use of movement corrections on the attenuation map has also been proposed in this connection, as is described, by way of example, by US 2008/0135769.
One problem with this procedure, however, is that, even during acquisition, the PET data changes forming the basis of the PET image data record can occur in the target area or even in the entire imaging area. Such changes can be caused, by way of example, by patient movements. Another significant interference factor, however, is the injection of contrast medium, by way of example magnetic resonance contrast medium, during the PET examination.
Such magnetic resonance contrast medium may, by way of example, be gadolinium chelates. Gadolinium (atomic number 64) significantly reduces X-rays and gamma rays and is also used as an X-ray contrast medium, cf. by way of example the article in The British Journal of Radiology, 73 (2000), 878-882 in this regard. The resulting gadolinium concentration is so low in large parts of an examined body that it may be ignored. Regions in which the contrast medium flows from the vein in concentrated form are a problem, however—in the case of injection into the arm veins typically the arm veins, the Vena brachiocephalica, the upper Vena cava and the heart. Furthermore, retrograde flow into the Vena jugularis may occur. This can lead to artifacts in the PET reconstruction of the adjoining areas (upper arm, throat, apex of the lung, heart, etc.).
The problem basically occurs with respect to the second imaging modality whenever a contrast medium is administered during the recording of PET data. This is less often the case with combined CT-PET devices since there the CT scans are usually carried out before the PET examinations. The problem of the other changes in the imaging area basically affects all devices in which PET is combined with an additional imaging modality.